+
+ +
+

compute rigid/local command

+
+

Syntax

+
compute ID group-ID rigid/local rigidID input1 input2 ...
+
+
+
    +
  • ID, group-ID are documented in compute command
    • +
  • rigid/local = style name of this compute command
    • +
  • rigidID = ID of fix rigid/small command or one of its variants
    • +
  • input = one or more rigid body attributes
    • +
+
+
+

Examples

+
compute 1 all rigid/local myRigid mol x y z
+
+
+
+
+

Description

+

Define a computation that simply stores rigid body attributes for +rigid bodies defined by the fix rigid/small command +or one of its NVE, NVT, NPT, NPH variants. The data is stored as +local data so it can be accessed by other output commands that process local data, such as +the compute reduce or dump local +commands.

+

Note that this command only works with the fix rigid/small command or its variants, not the fix rigid +command and its variants. The ID of the fix rigid/small command used to define rigid bodies must +be specified as rigidID. The fix rigid command is +typically used to define a handful of (potentially very large) rigid +bodies. It outputs similar per-body information as this command +directly from the fix as global data; see the fix rigid doc page for details

+

The local data stored by this command is generated by looping over all +the atoms owned on a processor. If the atom is not in the specified +group-ID or is not part of a rigid body it is skipped. If it is not +the atom within a body that is assigned to store the body information +it is skipped (only one atom per body is so assigned). If it is the +assigned atom, then the info for that body is output. This means that +information for N bodies is generated. N may be less than the # of +bodies defined by the fix rigid command, if the atoms in some bodies +are not in the group-ID.

+
+

Note

+

Which atom in a body owns the body info is determined internal +to LAMMPS; it’s the one nearest the geometric center of the body. +Typically you should avoid this complication, by defining the group +associated with this fix to include/exclude entire bodies.

+
+

Note that as atoms and bodies migrate from processor to processor, +there will be no consistent ordering of the entries within the local +vector or array from one timestep to the next.

+

Here is an example of how to use this compute to dump rigid body info +to a file:

+
compute 1 all rigid/local myRigid mol x y z fx fy fz
+dump 1 all local 1000 tmp.dump index c_1[1] c_1[2] c_1[3] c_1[4] c_1[5] c_1[6] c_1[7]
+
+
+
+

This section explains the rigid body attributes that can be specified.

+

The id attribute is the atomID of the atom which owns the rigid body, which is +assigned by the fix rigid/small command.

+

The mol attribute is the molecule ID of the rigid body. It should +be the molecule ID which all of the atoms in the body belong to, since +that is how the fix rigid/small command defines its +rigid bodies.

+

The mass attribute is the total mass of the rigid body.

+

There are two options for outputting the coordinates of the center of +mass (COM) of the body. The x, y, z attributes write the COM +“unscaled”, in the appropriate distance units (Angstroms, +sigma, etc). Use xu, yu, zu if you want the COM “unwrapped” by +the image flags for each atobody. Unwrapped means that if the body +COM has passed thru a periodic boundary one or more times, the value +is generated what the COM coordinate would be if it had not been +wrapped back into the periodic box.

+

The image flags for the body can be generated directly using the ix, +iy, iz attributes. For periodic dimensions, they specify which +image of the simulation box the COM is considered to be in. An image +of 0 means it is inside the box as defined. A value of 2 means add 2 +box lengths to get the true value. A value of -1 means subtract 1 box +length to get the true value. LAMMPS updates these flags as the rigid +body COMs cross periodic boundaries during the simulation.

+

The vx, vy, vz, fx, fy, fz attributes are components of +the COM velocity and force on the COM of the body.

+

The omegax, omegay, and omegaz attributes are the angular +velocity componennts of the body around its COM.

+

The angmomx, angmomy, and angmomz attributes are the angular +momentum components of the body around its COM.

+

The quatw, quati, quatj, and quatk attributes are the +components of the 4-vector quaternion representing the orientation of +the rigid body. See the set command for an explanation of +the quaternion vector.

+

The angmomx, angmomy, and angmomz attributes are the angular +momentum components of the body around its COM.

+

The tqx, tqy, tqz attributes are components of the torque acting +on the body around its COM.

+

The inertiax, inertiay, inertiaz attributes are components of +diagonalized inertia tensor for the body, i.e the 3 moments of inertia +for the body around its principal axes, as computed internally by +LAMMPS.

+
+

Output info:

+

This compute calculates a local vector or local array depending on the +number of keywords. The length of the vector or number of rows in the +array is the number of rigid bodies. If a single keyword is +specified, a local vector is produced. If two or more keywords are +specified, a local array is produced where the number of columns = the +number of keywords. The vector or array can be accessed by any +command that uses local values from a compute as input. See this section for an overview of LAMMPS output +options.

+

The vector or array values will be in whatever units the +corresponding attribute is in:

+
    +
  • id,mol = unitless
    • +
  • mass = mass units
    • +
  • x,y,z and xy,yu,zu = distance units
    • +
  • vx,vy,vz = velocity units
    • +
  • fx,fy,fz = force units
    • +
  • omegax,omegay,omegaz = radians/time units
    • +
  • angmomx,angmomy,angmomz = mass*distance^2/time units
    • +
  • quatw,quati,quatj,quatk = unitless
    • +
  • tqx,tqy,tqz = torque units
    • +
  • inertiax,inertiay,inertiaz = mass*distance^2 units
    • +
+
+
+

Restrictions

+
+
none
+
+ +
+ + +
+